A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives a fuel such as hydrogen gas and the cathode receives an oxidant such as oxygen or air. Several fuel cells are typically combined in a fuel cell stack to generate a desired amount of power. A typical fuel cell stack for a vehicle may include several hundred individual cells. Such a fuel cell stack is disclosed in commonly owned U.S. patent application Ser. No. 10/418,536, hereby incorporated herein by reference in its entirety.
The fuel cell stack includes a wet end adapted to receive the fuel, oxidizer, and cooling fluids, and a dry end having an insulation end plate unit. When producing the fuel cell stack, it may be necessary to pressurize the system to prepare the fuel cell stack for operation. The fuel cell stack is typically pressurized to test for leaks and to ensure that the stack will function efficiently. Over pressurization of the fuel cell stack is undesirable.
The basic process employed by a fuel cell is efficient, substantially pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators), and may be constructed to leave only heat and water as by-products. The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells depending upon the context in which it is used. The plurality of cells is typically bundled together and arranged to form a stack with the plurality of cells commonly arranged in electrical series. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a desired energy output level providing flexibility of design for different applications.
Different fuel cell types can be provided such as phosphoric acid, alkaline, molten carbonate, solid oxide, and proton exchange membrane (PEM), for example. The basic components of a PEM-type fuel cell are two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin catalyst layer. The electrodes, catalyst, and membrane together form a membrane electrode assembly (MEA).
In a typical PEM-type fuel cell, the MEA is sandwiched between “anode” and “cathode” diffusion mediums (hereinafter “DM's”) or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper, for example. The DM's serve as the primary current collectors for the anode and cathode as well as provide mechanical support for the MEA. The DM's and MEA are pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct current between adjacent cells internally of the stack in the case of bipolar plates and conduct current externally of the stack (in the case of monopolar plates at the end of the stack).
The secondary current collector plates each contain at least one active region that distributes the gaseous reactants over the major faces of the anode and cathode. These active regions, also known as flow fields, typically include a plurality of lands which engage the primary current collector and define a plurality of grooves or flow channels therebetween. The channels supply the hydrogen and the oxygen to the electrodes on either side of the PEM. In particular, the hydrogen flows through the channels to the anode where the catalyst promotes separation into protons and electrons. On the opposite side of the PEM, the oxygen flows through the channels to the cathode where the oxygen attracts the hydrogen protons through the PEM. The electrons are captured as useful energy through an external circuit and are combined with the protons and oxygen to produce water vapor at the cathode side.
Fuel cell stacks include unit cells and separators. Each fuel cell typically includes a solid polymer electrolyte membrane having a pair of electrode catalysts disposed on opposing surfaces. The fuel cell further includes a pair of collectors, each having a rigid body, the collectors in contact with respective electrode catalysts. Each of the separators includes a pair of pressure generating plates defining therebetween a pressure chamber, to which pressurized fluid is introduced. The pressure generating plates may be deformed by the pressurized fluid, and are pressed against adjacent collectors.
With current designs of fuel cell stacks, large volumes of hydrogen and air are mixed in the manifolds in the fuel cell stack, especially during start up. The mixing of hydrogen and air can result in a rapid production of water. The rapid production of water in the manifolds of the fuel cell stacks can cause over pressurization, resulting in an unpredictable deformation thereof.
It would be desirable to produce a fuel cell stack assembly having a pressure relief feature that relieves excess pressure from the fuel cell stack and facilitates a predictability of a deformation thereof.